1. Field of the Invention
The present invention relates to an aluminum nitride sintered product and a process for its production.
2. Discussion of Background
An aluminum nitride sintered product has a theoretical thermal conductivity as high as 320 W/mxc2x7K, and it is excellent also in the mechanical strength and electrical properties at a level of alumina. Accordingly, it has recently been widely used as a substrate material for a semiconductor power module such as GTO (gate turn off thyristor) or IGBT (insulted gate bipolar transistor) which requires high levels of electrical insulating properties and heat dissipation properties. As common properties of aluminum nitride sintered products which are industrially used for semiconductor power modules, the thermal conductivity is from 130 to 200 W/mxc2x7K, and the three-point bending strength (hereinafter referred to simply as the bending strength) is from 30 to 40 kg/mm2. For such a semiconductor power module, a copper-bonded substrate is widely used, wherein a copper sheet is bonded to an aluminum nitride substrate via an active metal layer or the like. In this copper-bonded substrate, there is a substantial difference in thermal expansion between aluminum nitride and the copper sheet, and cracks are likely to form in the aluminum nitride sintered product by heat treatment at the time of mounting electronic elements on the copper-lined substrate or by heat cycle exerted when it is used as a semiconductor power module, whereby the reliability as a semiconductor power module tends to be impaired. Accordingly, as an aluminum nitride substrate for such a semiconductor power module, one excellent in the bending strength, is required.
Aluminum nitride sintered products having the above properties are mass produced by the following method. Namely, a sintering aid such as yttrium oxide and an organic binder are blended to an aluminum nitride material powder, and the blend is molded into a molded product by e.g. a doctor blade method or a press-molding method. Then, this molded product is heated in air or in a nitrogen atmosphere to remove the binder, and then the molded product is sintered in a nitrogen atmosphere under an ambient pressure to obtain a sintered product. The thermal conductivity of an aluminum nitride sintered product depends largely on the amount of oxygen contained in the aluminum nitride crystal grains. Namely, by reducing the oxygen content, it is possible to obtain an aluminum nitride sintered product having a high thermal conductivity. According by using an aluminum nitride material powder having a small oxygen content or by incorporating carbon to an aluminum nitride material powder and reacting the carbon with the contained oxygen during the sintering process in a nitrogen atmosphere to remove the contained oxygen, sintered products showing a high thermal conductivity at a level of 200 W/mxc2x7K are mass-produced.
Further, many attempts have been made for the purpose of improving the bending strength of the aluminum nitride sintered product. For example, (a) it has been attempted to improve the strength by incorporating a Si component to control growth of aluminum nitride crystal grains during sintering thereby to form a sintered product having fine, dense aluminum nitride crystal grains (JP-A-6-329474,etc.), and (b) it has been attempted to increase the strength by dispersing fine particles (nano particles) of titanium oxide or the like in the crystal grains and grain boundaries of the aluminum nitride sintered product (JP-A-4-132666).
However, these methods for improving the strength have problems respectively and have not been practically employed. For example, in the above method (a), the Si component present in the aluminum nitride sintered product tends to lower the thermal conductivity, whereby it tends to be difficult to obtain high strength and high thermal conductivity at the same time. On the other hand, in the above method (b), the step for preparation of nano particles, the step for uniformly dispersing nano particles in the sintered product, etc., are cumbersome, and it has been difficult to adopt such a method widely on an industrial basis.
It is an object of the present invention to solve the above-mentioned problems of the prior art and to provide an aluminum nitride sintered product which can be produced constantly on an industrial basis, which has both high strength and high thermal conductivity and which is suitable for an aluminum nitride substrate for a semiconductor power module, and a process for its production.
The present invention provides an aluminum nitride sintered product which is made mainly of aluminum nitride and contains an yttrium compound in an amount of from 0.6 to 5 wt % as calculated as yttrium oxide, a vanadium compound in an amount of from 0.02 to 0.4 wt % as calculated as vanadium and carbon in an amount of from 0.03 to 0.10 wt % and which has a three-point bending strength of at least 45 kg/mm2 and a thermal conductivity of at least 150 W/mxc2x7K, wherein crystal grains of aluminum nitride have an average grain size of at most 5 xcexcm.
The present invention also provides a process for producing an aluminum nitride sintered product, which comprises molding a blend material prepared by blending a carbon material and a binder to a composition comprising from 0.6 to 5 wt % of yttrium oxide, from 0.02 to 0.4 wt %, as calculated as vanadium, of vanadium oxide and the rest being an aluminum nitride material powder, into a molded product of a predetermined shape, then heating the molded product in air to remove the binder from the molded product and then sintering the molded product having the binder removed therefrom, in a non-oxidizing atmosphere, wherein the content of the carbon material in the blend material is from 0.5 to 0.8 time by weight the amount of oxygen contained in the aluminum nitride material powder.
In the aluminum nitride sintered product of the present invention, if the average grain size of aluminum nitride crystal grains exceeds 5 xcexcm, the strength tends to deteriorate. The maximum grain size of the aluminum nitride crystal grains in the sintered product is preferably less than 15 xcexcm.
In the present invention, the average grain size of the aluminum nitride crystal grains, is determined as follows. With respect to a fracture surface of a sintered product, a SEM photograph with 2,000 magnifications is taken, and an optional linear line (a length of about 120 xcexcm) is drawn on the photograph. Then intersecting points of this linear line with grain boundaries of each crystal grain, are determined, and the length between the intersection points of each crystal grain is taken as the grain size of that crystal grain. Thus, the arithmetic average of grain sizes of the respective crystal grains is taken as the average grain size.
Further, the maximum grain size of crystal grains is determined in such a manner that with respect to a fracture surface of a sintered product, a SEM photograph with 1,000 magnifications, is taken, then, the maximum crystal grain is specified on the photograph (within an area of about 80xc3x97120 xcexcm) and the maximum diameter of that crystal grain is taken as the maximum grain size.
The yttrium compound serves as a sintering aid and will remain in the sintered product. If its content is less than 0.6 wt % as calculated as yttrium oxide, the product tends to be porous, and the strength of the sintered product tends to be low. On the other hand, if its content exceeds 5 wt % as calculated as yttrium oxide, the yttrium compound tends to partially agglomerate on the surface of the sintered product, and the surface roughness tends to increase. Such an yttrium compound may, for example, be yttrium oxide or yttrium aluminum oxide (Y4Al2O9).
The vanadium compound serves to promote sinterability at the time of sintering and will remain in the sintered product. The vanadium compound may, for example, be V2O3,V2O4 or V2O5. If its content is less than 0.02 wt % as calculated as vanadium, deformation of the sintered product is likely to occur, and if it exceeds 0.4 wt %, the aluminum nitride crystal grains tend to be too large, whereby the strength of the sintered product tends to be low.
Further, if the bending strength is less than 45 kg/mm2, the strength is inadequate, such being practically useless. If the thermal conductivity is less than 150 W/mxc2x7K, such a sintered product can not be used for an application where heat dissipation is required.
Such a sintered product is obtainable by blending yttrium oxide, vanadium oxide, carbon and a binder in predetermined amounts, respectively, to an aluminum nitride material powder and sintering the blend material.
The carbon material serves to improve the strength by suppressing growth of aluminum nitride crystal grains at the time of sintering. Such a carbon material is preferably employed in the form of a powder. The content of the carbon material is adjusted to be within a range of from 0.5 to 0.8 time by weight the amount of oxygen contained in the aluminum nitride material powder to be used. If the content of the carbon material is smaller than 0.5 time, the effect for suppressing the growth of aluminum nitride crystal grains tends to be inadequate, and if it exceeds 0.8 time, it tends to be difficult to obtain a dense sintered product, and the strength tends to be low. More preferably, it is within a range of from 0.6 to 0.74 time.
Such a carbon material may, for example, be one which is not substantially susceptible to oxidation or decomposition in the binder removal step of heating in air to remove the binder from the molded product. Specifically, such a carbon material is preferably carbon having a crystal structure which is not susceptible to oxidation or decomposition by heat treatment at 460xc2x0 C. in air. For example, graphite or carbon black which is not susceptible to oxidation or decomposition by heat treatment at 460xc2x0 C. in air, may be mentioned.
If carbon which is susceptible to oxidation or decomposition by heat treatment at a temperature of lower than 450xc2x0 C., e.g. from 380xc2x0 C. to lower than 450xc2x0 C., in air in the binder removal step, is used, there will be non-uniformity in the amount of carbon in the molded product after removal of the binder, whereby not only it becomes difficult to obtain a sintered product having the growth of aluminum nitride crystal grains uniformly suppressed, but also there will be non-uniformity in the sinterability of the molded product, whereby substantial warping or deformation of the sintered product is likely to take place during sintering. Further, if the particle size of the carbon powder is too large, it may leave pores in the sintered product, thus leading to deterioration of the strength. The maximum particle size of the carbon powder is preferably 10 xcexcm, more preferably at most 5 xcexcm.
Yttrium oxide will form a liquid phase in cooperation with an oxidized layer on the surface of the aluminum nitride material powder in the sintering process in a non-oxidizing atmosphere, and it serves to promote densification of the aluminum nitride sintered product. If the content of yttrium oxide is less than 0.6 wt %, a dense sintered product tends to be hardly obtainable, and if it exceeds 5 wt %, oozing out of the liquid phase to the surface of the sintered product tends to increase, whereby the strength tends to deteriorate, and non-uniformity in the strength tends to increase.
Vanadium oxide serves to promote sinterability thereby to suppress non-uniformity of the sinterability in the sintered product. If its content is less than 0.02 wt % as calculated as vanadium, its effect tends to be inadequate, whereby deformation during sintering can not be suppressed, and non-uniformity in strength is likely to result, and if it exceeds 0.4 wt %, aluminum nitride crystal grains are likely to grow too much, whereby the strength tends to deteriorate. Such vanadium oxide may, for example, be V2O3, V2O4 or V2O5.
The aluminum nitride sintered product of the present invention may further contain not more than 0.1 wt % of other components, as the case requires.
The aluminum nitride material powder to be used in the present invention may be one prepared by any method. However, preferred is one having an average particle size of at most 2 xcexcm. If the average particle size exceeds 2 xcexcm, the aluminum nitride crystal grains in the aluminum nitride sintered product tend to be too large, whereby the strength tends to deteriorate.
Further, if the amount of oxygen contained in the aluminum nitride material powder is too large, the reaction time to remove the contained oxygen tends to be long, such being uneconomical. Such an oxygen content is preferably at most 2 wt %, more preferably at most 1.5 wt %.
The binder will be removed from the molded product as it will be oxidized and decomposed when heated in air. The oxidation initiating temperature of aluminum nitride is about 460xc2x0 C., and as the binder, one which will be oxidized and decomposed when heated at a temperature lower than this temperature, will be employed. Specifically, one having a dispersant, a plasticizer or a solvent added to a resin such as polyvinyl butyral or an acrylic resin, may be employed. The content of the binder is suitably selected within a range wherein the molding operation to obtain a molded product is easy, and removal of the binder in the binder removal step, is easy.
A carbon material and a binder are blended to a composition comprising an aluminum nitride material powder, yttrium oxide and vanadium oxide in the above-mentioned proportions, to obtain a blend material. This blend material is molded into a molded product of a predetermined shape such as a plate shape. As such a molding method, press-molding or doctor blade molding may, for example, be employed.
Then, the molded product is heated in air to remove the binder from the molded product. The temperature for this heating is a temperature lower than 460xc2x0 C. as mentioned above. Further, in consideration of the productivity, it is preferably within a range of from 400 to 460xc2x0 C. This heating time varies depending upon the thickness of the molded product, the amount of the binder, etc., and it is about 60 minutes in the case of a molded product having a thickness of about 1 mm.
Then, the molded product having the binder removed therefrom, is heated to a temperature of from 1,700xc2x0 C. to the sintering temperature in a non-oxidizing atmosphere over a period of from 1 to 5 hours and then sintered by maintaining it at the sintering temperature for from 1 to 5 hours. This sintering temperature is within a range of from 1,900 to 2,000xc2x0 C. The non-oxidizing atmosphere may, for example, be nitrogen gas, helium gas, argon gas or a vacuumed atmosphere. If the sintering temperature is lower than 1,900xc2x0 C., it tends to be difficult to obtain a dense sintered product, and if it exceeds 2,000xc2x0 C., the growth of the aluminum nitride crystal grains tends to progress, whereby it tends to be difficult to obtain aluminum nitride crystal grains having an average grain size of at most 5 xcexcm.
The aluminum nitride sintered product thus obtained, has a fine structure such that the average grain size of aluminum nitride crystal grains is at most 5 xcexcm, and has high thermal conductivity and high strength such that the thermal conductivity is at least 150 W/mxc2x7K and the bending strength is at least 45 kg/mm2, and it is particularly suitable as a substrate for a semiconductor power module.